Anoxygenic Photosynthesis

Detailed structural information on the more ancient reaction centers of non-oxy-genic bacteria is only available for type-II reactions centers from purple bacteria [32, 33] (see Chapter 12). None of the ancient type-I photosystems from heliobac-teria or green- sulfur bacteria have been crystallized so far, but biochemical, biophysical, and molecular biological studies shed light on the structural composition and organization of the ancient type - I reaction centers and the evolution of all photosystems as described in Chapter 13 .

The structure of the purple bacterial reaction center is shown in Figure 1.3. The left side shows the structure of the reaction center; the right shows the structure of the supercomplex of the reaction center with the LH1 ring, described in Chapter 14. In addition, the LH2 ring is shown; it serves as a mobile peripheral antenna in purple bacteria.

The reaction center of purple bacteria represents a type-II reaction center. Chapter 12 describes in detail its structure, function, and evolution. The successful crystallization of the purple bacteria reaction center by Michel and Deisenhofer in 1985 [34] opened up the new research field of membrane protein structure determination. The purple bacterial photoreaction center was the first membrane protein, to have been crystallized, and led to the Nobel Prize in 1988 for their work.

The reaction center consists of three subunits: L, M, and H. Some purple bacteria (as e.g. R. viridis) also contain a cytochrome, bound at the outer (luminal) side of the complex. The L and M subunits form the central core of the reaction center and bind all the cofactors. The H subunit contains only one transmembrane helix and a larger cytosolic (stromal) domain that stabilizes the reaction center. The electron transport chain of the purple bacterial reaction centers is very similar to the electron transport chain in Photosystem II. It consists of a special pair of bacteriochlorophylls, P, which form the primary electron donor. When P is excited to P*, an electron is transported along the L branch of the electron transport chain; this involves an accessory chlorophyll (accChl), a pheophytin (Pheo), a tightly bound quinone (QA) , and the mobile quinone QB . After two charge separation events the doubly reduced Qb takes up two protons from the cytosolic (stromal) side of the membrane and leaves the binding pocket to transfer the electron to the bc complex. The bc complex shows homologies to the cytochrome bf complex in plants and pumps protons across the membrane. P+ is reduced by a soluble cyto-chrome which shuttles electrons between the RC and the cytochrome bc complex. Note that no NADPH is produced in this process, but the charge separation leads to the transmembrane proton motive force that drives ATP synthesis.

Structural information on the photosystems from heliobacteria and green sulfur bacteria, which contain a type-I reaction center, is not yet available as none of these photosystems have been crystallized so far. It should be noted that, in this book, the term photosystem is used for all type - I reaction centers to reflect that they contain a reaction center and a core antenna system; this defines them as a photosystem. A description of the present knowledge of the ancient type-I photosystems is given in Chapter 13. which also contains a detailed discussion of the potential structure and function of the most ancient reaction center, the so called ur - reaction center.

The anoxygenic type-I photosystems contain homodimeric reaction centers. The core is formed by a homodimer of one large subunit (subunit A), which resembles the core subunits PsaA/B of PSI and harbors all the membrane intrinsic cofactors of the electron transport chain and core antenna chlorophylls. The electron transport chain consists of six chlorophylls, one 4Fe4S cluster, FX, and very likely two quinones. The properties of the electron transport chain are similar to the electron transport chain in PSI, except that the role of the quinones in electron transport is still under debate -35, 36]. Recent evidence may indicate that a quinone is present and involved in the electron transfer reactions but may be only weakly bound and therefore easily lost during purification.

The type-I reaction centers also contain a stromal subunit (subunit B) that contains two 4Fe4S clusters that resemble FA and FB in PSI. However, this subunit is larger than subunit PsaC in oxygenic photosynthesis and is very loosely bound to the core. In addition to these two core subunits, the Photosystem from green sulfur bacteria also contains a membrane bound cytochrome subunit (subunit C).

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